body {
  font-family: system-ui, sans-serif;
  margin: 2rem auto;
  max-width: 480px;   /* 限制宽度，天然单列 */
  background: #f7f7f7;
}
h1 {
  text-align: center;
  margin-bottom: 1.5rem;
}
section {
  margin-bottom: 1.2rem;
  background: #fff;
  border: 1px solid #ddd;
  padding: .8rem 1rem;
  border-radius: 4px;
}
label {
  margin-right: 1rem;
}
input[type="number"] {
  width: 100px;
  margin-left: .5rem;
}
.row {
  margin: .6rem 0;
}
.center {
  text-align: center;
}
button {
  padding: .4rem 1.2rem;
}
.val {
  font-weight: bold;
  color: #0057d9;
}
.hidden {
  display: none;
}